Marine Landforms: Erosional and Depositional

• Marine or coastal landforms are the erosional and depositional features shaped by sea waves, currents and tides along the belt where land meets sea.
  • Waves do almost all the work; currents and tides redistribute what the waves loosen.
• The coast is among the fastest-changing landscapes on Earth: a cliff can retreat, or a spit grow, within a human lifetime.
• India’s coastline, re-measured in 2025 at a finer map scale with its islands fully counted, is about 11,099 km long, with cliffs, lagoons, spits, barrier islands and deltas along it.

Agents and Processes of Marine Work

Sea Waves

• Sea waves are undulations of sea water with crests and troughs, generated mainly by friction of wind on the water surface.
  • At their source they form swell: low, broad, regular, rounded waves.
  • Wave height depends on wind speed, wind duration and fetch (the stretch of open water the wind blows over).
  • Wavelength is the distance between two crests and wave period the time to cover it; longer waves have lower frequency and travel faster.
• Only the wave form moves forward, not the water. Particles move in circular orbits that shrink with depth and flatten into a back-and-forth motion near the bottom.
• Near the coast a wave “feels bottom” at a depth of about half its wavelength.
  • Friction slows it, the wavelength shortens and the height grows until the crest topples as a breaker.
  • The plunge line, where depth roughly equals wave height, is where waves become geomorphic agents.
• Breakers are spilling (crest spills down the front, on gentle shores), plunging (crest curls and falls, most erosive) and surging (low waves rush up steep beaches).
• Water rushing up the beach is the swash (uprush or surf); water returning seaward is the backwash.
  • The seaward return along the bottom is the undertow; narrow, fast seaward jets through the surf are rip currents, a hazard to bathers.
• Wave base is the depth, about half the wavelength, below which orbital motion dies out.
  • Waves cannot erode or stir sediment below it, which limits how deep shore platforms are cut; long storm waves have a deeper storm wave base.
  • The belt between the plunge line and the shore, dominated by broken water, is the surf zone.

• Oscillatory waves (deep water): particles return almost to their original position. Translatory waves (shallow water): water moves forward with the wave form.

Feature	Constructive waves	Destructive waves
Form	Low, long wavelength, low frequency	High, steep, short wavelength, high frequency
Breaking	Spill	Plunge
Swash vs backwash	Strong swash, weak backwash (water soaks into beach)	Weak swash, powerful backwash
Net effect	Build beaches and berms	Comb material seaward
Typical weather	Calm	Storms, monsoon

Currents, Tides and Tsunamis

• Wave refraction: waves bend to align with the coast in shallow water, concentrating energy on headlands and spreading it in bays.
• Longshore (littoral) currents flow parallel to the coast when waves strike it obliquely.
  • With the zigzag swash and backwash of oblique waves they drive longshore drift, the main carrier of beach sediment.
  • Along India’s east coast the net drift is northward, so spits and bars there tend to grow northward.
• Tides shift the zone of wave attack up and down the shore and scour inlets and creeks.
  • The Gulf of Khambhat, with an average range of about 10 m near Bhavnagar, the highest in India, is fringed by vast mudflats.
• Tsunamis and storm surges can remodel beaches, dunes and lagoon mouths in hours, as in 2004 on the Tamil Nadu coast.
• Wave reflection: waves striking plunging cliffs or sea walls bounce back seaward instead of breaking.
  • Reflected and incoming waves combine into standing waves, which scour the sea bed and lower the beach at the wall’s toe.

Coast, Shore and Their Zones

• The shore lies between high tide water (HTW) and low tide water (LTW); the shoreline is the water-land boundary at any moment.
• The coast is the land immediately behind the shore; the coastline is the cliff line or margin of land above the sea.

Zone	Extent
Backshore	From the reach of storm waves to the cliff base
Foreshore	Between low and high tide water
Offshore	Shallow sea floor seaward of low tide

Processes of Marine Erosion

• Breaking waves turn potential energy into kinetic energy, and the swash strikes the coast through four processes.
• Hydraulic action: the impact of breaking water.
  • Storm waves strike with pressures of tens of tonnes per square metre, widening joints and dislodging blocks of several tonnes (wave quarrying).
  • Air in cracks is compressed by the swash and released by the backwash; the repeated pressure change shatters rock.
• Corrasion or abrasion: waves armed with sand, pebbles and boulders grind and undercut the cliff base.
• Attrition: moving fragments collide and wear one another down into pebbles and sand.
• Corrosion or solution: sea water dissolves carbonate rocks such as limestone and chalk.
• Coastal weathering helps: wetting and drying, salt crystallisation, frost on cold coasts and slumping of cliff faces.

Factors Controlling Coastal Erosion

• Wave energy and duration, rock type and jointing, the supply of tools, and stability of sea level.
• Depth at the cliff foot: deep water reflects waves; a shallow platform makes breakers hammer the cliff.
• Orientation: on the basalt coast of Maharashtra waves strike joints transversely and have cut coves, caves, stacks and inlets, while the basaltic south Kathiawar coast, attacked parallel to its trend, has few.

Marine Transportation and Profile of Equilibrium

• Waves carry load by solution, suspension, saltation and traction.
• Transport runs both ways: backwash carries material seaward, swash brings it back, and longshore currents move it along the shore.

• A profile of equilibrium is reached when the shore slope lets incoming sediment be carried away as fast as it arrives.
  • A slope too steep is lowered by destructive waves; one too gentle is built up by constructive waves.

Erosional Landforms

Headlands and Bays

• Where hard and soft rocks alternate across the shoreline, soft bands are cut back into bays and resistant bands stand out as headlands.
• Refraction focuses attack on headlands (cliffs, platforms, caves) and leaves bays sheltered for bay-head beaches.

Cliffs and Wave-cut Platforms

Sea Cliffs

• A sea cliff is a steep, almost vertical rocky coast, often with an overhanging crest.
• Its form depends on lithology and structure and on the balance between marine erosion at the base and subaerial weathering of the face.
  • Faster marine erosion gives a vertical, overhanging cliff; faster weathering gives a gentler, degraded slope.
• Recession: waves cut a wave-cut notch between tide levels; the crest overhangs and collapses; backwash clears the debris; the cliff retreats landward.
  • The rate depends on rock type, cliff height, orientation, wave energy, offshore depth and removal of debris.

Cliff type (André Guilcher, 1958)	Rock	Form
Resistant	Chalk, horizontal sandstone	Steep, near-vertical
Weak	Clay, shale	Gentle, slumping
Composite	Chalk over clay; sandstone with shale	Stepped
Complex	Mixed structure and processes	Irregular

• Indian example: the Varkala cliffs of Kerala, up to about 40 m high in Mio-Pliocene sandstones and clays, are a National Geological Monument placed on UNESCO’s tentative list in 2025; building on the cliff edge is speeding their erosion.

Wave-cut Platforms

• A wave-cut (shore) platform is a gently sloping, slightly concave rock surface at the cliff foot, left as the cliff retreats.
  • Wide platforms form on thin-bedded, closely jointed rocks; resistant rocks give narrow, higher ones.
  • Storm-wave quarrying and water-level weathering shape them.
• Zones: supralittoral (spray zone), mesolittoral (between tides), sublittoral (below low tide).
• Types: inclined, stepped (tropical, small tidal range), storm-wave and solution platforms (on carbonate rocks).
• Debris swept off the platform builds a wave-built terrace beyond it.

Caves, Arches, Stacks and Stumps

• These develop in sequence on headlands:
  • Sea caves: joints widened by hydraulic action and abrasion; common in limestone.
  • Natural arch: a cave cuts through the headland, or two caves meet from opposite sides.
  • Stack: the arch roof collapses, leaving an isolated pillar (chimney rock, needle, skerry), such as the Old Man of Hoy (137 m) in Orkney.
  • Stump: the undercut stack collapses to a low rock, often covered at high tide.
• Indian example: the natural sea arch at Mangamaripeta beach between Visakhapatnam and Bheemunipatnam.

Blowholes and Geos

• Storm waves surging into a cave force compressed air and spray up through a joint in the roof, opening a blowhole or gloup.
• The roof weakens and collapses, leaving a long, narrow, steep-sided inlet called a geo.

Coves, Chasms, Creeks and Inlets

• A cove is a small, rounded bay with a narrow entrance, hollowed out of soft rock behind a breached resistant band.

• Chasms are narrow, deep clefts cut along vertical planes of weakness; lateral erosion later widens them into bays.

• A coastal creek is a narrow, sheltered tidal waterway into land or marsh, like the creeks of the Konkan and Sir Creek in the Rann of Kachchh.
• An inlet is a long, narrow indentation of the shoreline; a tidal inlet is a gap through a barrier linking a lagoon to the sea.

Erosional Coasts of India

• Best seen on the Konkan and Karnataka coast between Mumbai and Mangalore: cliffed headlands, coves and pocket beaches, with headlands near Ratnagiri carrying cliffs 45–90 m high.
• On the east coast they cluster around Visakhapatnam, where the Eastern Ghats meet the sea.
• Shoreline change: between 1990 and 2018 about 33.6% of India’s shoreline eroded, 26.9% accreted and 39.6% stayed stable (National Centre for Coastal Research).
  • At Satabhaya on the Odisha coast the sea has swallowed farmland and homes, and villagers were resettled inland.
  • Breakwaters block longshore drift: sand piles up south of Chennai port (Marina Beach) while beaches to the north erode.

Depositional Landforms

• Deposition occurs where waves lose energy: in sheltered bays, behind islands, where the coast changes direction and at river mouths.
• Sediment comes from eroded cliffs, rivers, cliff landslides, offshore scour and older beaches.

Beaches

• A beach is a temporary, wedge-shaped accumulation of sand, shingle or cobbles between low tide and the reach of storm waves: sediment in motion along the shore.
  • Constructive waves build it in calm weather; storm waves cut it back.
• Profile:
  • Upper beach: pebbles and cobbles, slope often 10°–20°.
  • Lower beach: sand, slope about 2° or less, with ripples, ridges and runnels.
  • Storm beach: a semi-permanent ridge above the highest spring tides.
  • Berms: low shore-parallel ridges marking successive high-tide and storm levels.
  • Beach cusps: small crescent embayments between shingle horns.

Beach type	Material	Profile	Setting
Sand beach	Sand (1/16–2 mm)	Gentle, wide	Low-energy bays
Shingle beach	Pebbles (2–64 mm)	Steep, with berms	Below eroding cliffs
Boulder beach	Cobbles, boulders	Steep	High-energy cliff foot

• Progradation is the seaward growth of a beach; retrogradation is its narrowing by erosion.
• Indian example: long sandy east-coast beaches (Marina, Puri, Digha) against short pocket beaches between Konkan headlands.

Bars and Barriers

• Bars are ridges of sand and shingle built by waves, usually parallel to the coast; larger ones are barriers.
  • They start as submerged shoals that grow upward until they emerge.
• Offshore (longshore) bars lie parallel to the coast, unattached to land.
• Barrier islands are long, narrow sand islands parallel to the coast, backed by a lagoon and cut by tidal inlets.
  • They form by upward growth of offshore bars, by breaching of spits, and by drowning of coastal dune ridges during post-glacial sea-level rise.
  • They shield the mainland from storm waves and surges and shelter lagoons, mangroves and fisheries, but migrate landward as the sea rises.
  • Indian example: Sriharikota, a spindle-shaped barrier island between Pulicat Lake and the Bay of Bengal.

Spits and Hooks

• A spit is a ridge of sand or shingle attached to land at one end, the other projecting into the sea.
  • It forms where longshore drift meets a bend in the coast or a river mouth and deposition continues in the old direction.
  • The sheltered water behind it silts up into salt marsh or mudflat.
• Storm waves or a shift in wind bend the tip landward into a hook; repeated bending gives a compound hook.
• Indian examples: the spit enclosing Chilika, the spit near Kalingapatnam, Hope Island growing north from the Godavari delta to shelter Kakinada Bay, and the spits enclosing Vembanad Lake at Kochi.

Bay Bars, Lagoons and Loops

• Bars may form at the head (bay-head bar), middle (mid-bay bar) or mouth (bay-mouth bar) of a bay.
• A spit that grows across a bay to join two headlands becomes a bay bar; the water trapped behind is a lagoon, which slowly fills into marsh.
• A spit bent back to the land by opposing currents encloses a lagoon as a loop; around an island it forms a looped bar.
• Indian examples:
  • Chilika (Odisha), Asia’s largest brackish-water lagoon and one of India’s first two Ramsar sites (1981); a new mouth was cut through its barrier in 2000 to restore tidal flushing.
  • Pulicat (Andhra Pradesh–Tamil Nadu), the second largest, and the kayals (backwaters) of Kerala.

Tombolos, Connecting Bars and Winged Headlands

• A connecting bar joins two headlands or two islands.
• A tombolo joins an island to the mainland or a headland, built where waves refracted round the island meet and drop their load in its lee.
• A winged headland carries pebble bars on both flanks.
• Indian examples:
  • Small tombolos on the Konkan coast between Ratnagiri and Malvan.
  • Rameswaram: Pamban Island ends in the Dhanushkodi spit, and Adam’s Bridge (Ram Setu) continues it as a chain of shoals to Mannar Island; satellite laser mapping in 2024 found about 99.98% of it submerged, a former land link now drowned.

Coastal Wetlands and Sabkhas

• Coastal wetlands are flat, marshy, low-relief tracts formed behind spits, bars and barrier islands, where fine silt and clay settle in quiet, tidally flooded saline or brackish water.
  • In the humid tropics they carry mangrove swamps; in cooler latitudes salt-tolerant grasses form salt marshes; unvegetated parts are tidal mudflats.
  • Mangrove roots trap sediment and build the swamp seaward, and the forests absorb storm surges.
  • Indian examples: the Sundarbans of the Ganga–Brahmaputra delta, the world’s largest mangrove forest; Bhitarkanika (Odisha); Pichavaram (Tamil Nadu); the mangroves and mudflats of the Gulf of Kachchh.
• Sabkhas are flat, barren, salt-encrusted coastal plains of hot arid coasts, lying just above normal high-tide level; they are also called coastal salt flats.
  • Seawater driven in by spring tides and storms, and brine drawn up by capillary rise, evaporate and leave gypsum, anhydrite and halite crusts over algal mats and fine sediment.
  • Examples: the Abu Dhabi (UAE) sabkha, the classic case, and the coasts of Qatar, Egypt and Baja California; India’s Rann of Kachchh, a seasonally flooded salt flat, is its closest equivalent.

Coastal Dunes

• On-shore winds blow dry beach sand landward into coastal dunes and dune belts.
  • Advancing dunes bury fields, roads and villages; sand-binding grasses and casuarina are planted to fix them.
  • Dunes store sand and buffer storms; mining them speeds shoreline retreat.
• Indian example: the Erra Matti Dibbalu red sand mounds near Bheemunipatnam, a geo-heritage site placed on UNESCO’s tentative list in 2025.

Classification of Coasts

• Coasts resist neat classification because sea level has repeatedly risen and fallen, most coasts are compound, and schemes use different bases (genesis, process, energy, tectonics).

Johnson’s Genetic Classification (1919)

• Douglas Wilson Johnson (1919) classified shorelines by emergence or submergence relative to sea level and by the nature of the land before the change.

Type	Origin	Forms and examples
Submergence	Sea-level rise (deglaciation) or land subsidence	Ria (drowned river valley); fiord (drowned glacial trough, Norway)
Emergence	Sea-level fall or uplift	Straight coast, gentle plain, offshore bars, lagoons
Neutral	Built by deposition or new land	Delta, alluvial plain, outwash plain, volcanic, coral reef, fault
Compound	Evidence of both	Norwegian coast

• A ria widens seaward like a funnel; a fiord is a deep, steep-walled drowned glacial trough.

• Merits (John B. Lucke, 1938): simple, genetic, logical and easy to apply.
• Criticism (Francis Parker Shepard):
  • Every coast has been both submerged and emerged during Quaternary sea-level changes, so nearly all are compound.
  • Deltas are not “neutral”: the Mississippi delta records both.
  • It overlooks eustatic changes caused by glaciation and deglaciation.

Shepard’s Classification (1937, revised 1948)

• Shepard’s 1937 scheme was faulted by Lucke for ignoring shorelines, relying on charts and lacking an evolutionary basis; the 1948 revision divides coasts by the agents that shaped them.

Primary (youthful): non-marine agents	Secondary (mature): marine agents
Drowned river (ria) and drowned glaciated (fiord) coasts	Shorelines straightened by marine erosion
Subaerial deposition: delta, alluvial plain, moraine, drumlin, aeolian, vegetation	Irregular shorelines cut by marine erosion
Volcanic: lava-flow and collapsed-cone coasts	Shorelines straightened or prograded by deposition
Diastrophic: fault-scarp and folded coasts	Offshore bars and spits; coral reef coasts

Later Classifications

• John Lloyd Davies (1964) used tidal range (microtidal <2 m, mesotidal 2–4 m, macrotidal >4 m) and wave energy:
  • High-energy coasts face strong swell and storms: cliffs, platforms, erosion.
  • Low-energy coasts lie in sheltered seas or behind wide shallow shelves: mudflats, marshes, mangroves, deposition.
  • Coral coasts are biogenic: fringing, barrier and atoll reefs in warm, clear, shallow tropical water.
• Douglas L. Inman and Carl E. Nordstrom (1971) classified coasts by plate tectonics:
  • Collision coasts (converging margins): narrow shelf, cliffs, young mountains.
  • Trailing-edge coasts (passive margins): wide shelves, plains, deltas, barriers; both Indian coasts belong here.
  • Marginal-sea coasts facing island arcs: sheltered and depositional.

Marine Cycle of Erosion

• Douglas Wilson Johnson (1919) extended the Davisian cycle of erosion to coasts: initial, youth, maturity and old stages, run separately for shorelines of submergence and emergence.
• It assumes a stable sea level and crust after the initial change, with waves and currents as the dominant agents.

Cycle on a Shoreline of Submergence

• Initial: drowning produces a highly irregular ria or fiord coast of bays, headlands, inlets and islands.
• Youth: notches and low cliffs; caves, arches, stacks multiply; wave-cut platforms widen as cliffs retreat.
  • Late youth: platforms are widest; bars, spits, hooks, loops and tombolos close bays into lagoons.
• Maturity: headlands cut back, bays filled, profile of equilibrium reached; a straight, regular coast.
• Old: land worn to near sea level; only theoretical.

Cycle on a Shoreline of Emergence

• Initial: a straight, gently sloping coastal plain; waves break offshore, cut small cliffs called nips, and build submarine bars.
• Youth: bars emerge and join into offshore bars enclosing lagoons, breached by tidal inlets; lagoons fill into marshes.
  • Late youth: storm waves shift material from the seaward to the landward face, so bars migrate coastward and lagoons narrow.
• Maturity: bars and lagoons are destroyed; the platform is cut to wave base; a straight, steeper coast.
• Old: deduced only in theory.

Stage	Submergence coast	Emergence coast
Initial	Indented ria or fiord coast	Straight plain, nips, submarine bars
Youth	Cliffs, caves, arches, stacks	Offshore bars, lagoons, tidal inlets
Late youth	Widest platforms, bays closed by bars	Bars migrate landward, lagoons shrink
Maturity	Straight coast, equilibrium profile	Bars gone, steeper coast
Old	Theoretical	Theoretical

Evaluation and Current View

• Strength: a simple genetic sequence explaining why indented coasts straighten with time.
• Criticism:
  • Sea level has never been stable long enough: the post-glacial rise slowed only about 6,000–7,000 years ago, and endogenic forces keep raising and lowering coasts.
  • Most coasts are compound; the scheme is descriptive and ignores sediment supply, tidal range, climate, corals and mangroves.
• Current view: coasts are studied as littoral cells with sediment budgets, in which sand is supplied, moved along the shore and lost.
  • Sea-level rise (about 0.20 m globally between 1901 and 2018, and accelerating) is driving erosion and landward roll-back of barriers.
  • Human interference (ports, breakwaters, sand mining, dams trapping river sediment) now shapes many Indian shorelines as strongly as waves do.